Fiber-reinforced epoxy composite joints

ABSTRACT

An improved manufacture including a fiber-reinforced epoxy composite joint, and the method of producing same, in which the fibers of each of the composite adherends pass through and are intermeshed across the joint bondline. A scarfed mating surface is prepared on the end of each of the adherend elements-to-bejoined, after which a chemical etching of the mating ends to obtain exposed lengths of filaments or fibers is provided. A sheet of unsupported adhesive material is inserted between the scarfed and etched mating surfaces prior to their layup, after which heat and pressure are applied for bonding the joint. The exposed fibers of each element cross the bondline to intermesh with those of the other element during the melting of the unsupported adhesive. This process increases the joint efficiency to 83 percent of the efficiency of a non-joined element.

1111' 3,885,071 14 1 May 20, 1975 FIBER-REINFORCED EPOXY COMPOSITE JOINTS [75] Inventors: Leiv H. Blad, Van Nuys; Robert F.

Warkow, Granada Hills, both of Calif.

[73] Assignee: Lockheed Aircraft Corporation,

Burbank, Calif.

[22] Filed: Aug. 10, 1972 [21] Appl. No.: 279,703

[52] US. Cl. 428/60; 156/3; 156/159;

156/304; 156/306; 428/33; 428/295 [51] Int. Cl B32b 3/10; B32b 5/12 [58] Field of Search 161/36, 37, 38, 53, 143,

FOREIGN PATENTS OR APPLICATIONS Germany 156/304 459,331 1/1937 United Kingdom 156/304 902,614 8/1962 United Kingdom 161/145 Primary Examiner-George F. Lesmes Assistant Examiner-Charles E, Lipsey 1 Attorney, Agent, or FirmBilly G. Corber; Frank L. Zugelter; Lowell G. Turner [5 7 ABSTRACT An improved manufacture including a fiber-reinforced epoxy composite joint, and the method of producing same, in which the fibers of each of the composite adherends pass through and are intermeshed across the joint bondline. A scarfed mating surface is prepared on the end of each of the adherend elements-to-bejoined, after which a chemical etching of the mating ends to obtain exposed lengths of filaments or fibers is provided. A sheet of unsupported adhesive material is inserted between the scarfed and etched mating surfaces prior to their layup, after which heat and pressure are applied for bonding the joint. The exposed fibers of each element cross the bondline to intermesh with those of the other element during the melting of the unsupported adhesive. This process increases the joint efficiency to 83 percent of the efficiency of a non-joined element.

3 Claims, 7 Drawing Figures 1 FIBER-REINFORCED EPOXY COMPOSITE JOINTS BACKGROUND OF THE INVENTION 1". Field ofthe Invention The field of art to which the invention is most likely to pertain is located in a class of art generally relating to fiber-reinforced epoxy composites. Class 156, Adhesive Bonding and Misc. Chemical Manufacture, and Class 74, Machine Elements and Mechanisms, U.S. Patent Office Classifications, may be the applicable general areas of art in which the claimed subject matter of the type involved here may be classified.

2. The Prior Art Another object of the invention is to increase the joint efficiency between two joined fiber-reinforced epoxy composite elements, heretofore never having Examples of the prior art in the arts to which this in- I vention most likely pertains are U.S. Pat. Nos. 2,309,305; 2,429,119; 3,101,290; 3,194,702; 3,218,211; 3,481,807; 3,563,829; 3,589,959; 3,419,449; and 3,546,054.

3. Problems in the Prior Art Previously the faying surfaces of fiber-reinforced epoxy composite adherends have generally been prepared by cleaning techniques such as sandblasting, tear plies, mechanical abrading, or solvent cleaning. Application of a supported adhesive to such faying surfaces was then undertaken, and the step of bonding the ad herends would follow.

More particularly, heretofore, the composite joints of laminated epoxy matrices reinforced with, say, for example, graphite and boron fibers, have been relatively weak, for the reason that the supported adhesive contained a fabric to control the thickness of the bondline, and although the fibers of each element would be introduced to the bondline of the joint, they could never pass through such fabric to project through such bondline to effect an intermeshing or contiguity with the fibers of the other element contributing to the joint. The preparation of the elements forming the joint by, say, machining to effect scarfed surfaces, would never provide other than the mere tips of any exposed fibers to be introduced into, but never past, the bondline. As a result, the joint was severely restricted in transferring structural loads therethrough.

Another drawback in the use of epoxy composites heretofore, besides cost, has been the inability to produce joints with high efficiencies without adding new material. The addition of the new adhesive and reinforced laminate would severely affect the weight of the joint.

This invention substantially improves such load transferring capabilities and efficiencies, by strengthening the heretofore relatively weak joints by means of the intermeshing and contiguous lengths of fibers in the one scarfed element with similar lengths of fibers in the second scarfed element. The advantage of lightweightness is obtained with the resulting increased strength obtained at the joint.

SUMMARY OFTI-IE INVENTION This invention relates to the-fabrication of joints formed of fiber-reinforced epoxy composites, and is more particularly concerned with a method of producing an improved epoxy composite joint having increased joint strength, and to the improved manufacture itself.

An object of this invention is to provide a fiberreinforced epoxy composite joint having the characteristics of high strength and high fatigue resistance.

been realized.

Another object of this invention is to eliminate required tight tolerances for scarfing Purposes, demanding costly machining techniques, which heretofore have been utilized in known processes to form a fiberreinforced epoxy composite joint of adequate strength.

Another object of this invention is to provide a fiberreinforced epoxy composite joint having improved or increased load carrying and transfer characteristics.

Another object of this invention is to provide the facility of changing the dimensions of a manufacture (i.e., re-working it) by utilization of the subject matter of the invention.

A further object of the invention is to provide an economically producible joint for fiber-reinforced epoxy composite materials or elements.

Another object of this invention is to reduce the extent of the fabricating required of such a joint by eliminating the addition of new material for reinforcing the joint, thus reducing joint weight and economical costs, while providing lightweightness as a result of the increased strength of the joint.

A further object of this invention is to provide a repair technique which is now available at the point of inplace installation, as compared to removal and repair thereafter, all leading to economies in time, material and labor, and transportation.

A further object of this invention is to provide an improved joint between a fiber-reinforced epoxy composite element and metallic elements such as aluminum, titanium and steel, and the manufacture resulting therefrom.

Another object of this invention is to provide a novel manufacture by utilization of the disclosed process.

These and other objects and advantages of this invention will become more fully apparent upon a complete reading of the following description, the accompanying drawing, and the appended claims hereto.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a pair of fiberreinforced epoxy composite elements to be joined together.

FIG. 2 is a perspective view of such elements after their sides or edges to-be-joined have been scarfed.

FIGS. 3, 4 and 5 illustrate, respectively, perspective, elevational and end views of such elements after the step of chemical etching.

FIG. 6 is a perspective view of the scarfed and etched sides, ends or edges of such elements prior to joining same together.

FIG. 7 is a perspective view of such elements immediately prior to the laying up step and after introduction of unsupported adhesive material between the elements-to-be-joined.

DESCRIPTION OF THE PREFERRED .EMBODIMENT I In practicing this invention, in order to acquire the desired strength and joint efficiency in an epoxy composite manufacture, the faying surfaces of adherends are first developed by preparing scarfed end portions on the fiber-reinforced materials-to-be-joined, and thereafter introducing such scarfed end portions to a chemical etchant whereby exposed lengths of fibers or filaments are obtained on the scarfed end portions. The fibers of the scarfed end portions of the adherends are exposedto a depth of, say, 0.010 inch, as a result of such etching or chemical removal of the resin in the material. Preferably, the range of length is 0.0l to 0.040 inch, however, the length may be longer. Afterwards, the scarfed end surfaces to-be-mated are cleaned; i.e., the etching acid is neutralized and any residual debris and fluids removed. They are then joined together with an unsupported adhesive material which comes in the form of a film or sheet. The unsupported adhesive is introduced to the ends prior to joining them together by utilization of known layup techniques, mechanical or otherwise. With heat and pressure applied during a subsequent bonding stage, such film melts, and the lengths of the fibers intermesh, resulting in physical contiguity of the fibers of the length of the etched portion of the adherends. The bonding of the adherends effects an improved epoxy composite joint.

As an illustrative embodiment of the subject matter of the invention, the preparation for, production of, and a manufacture including, an epoxy composite joint in accordance therewith and the necessary materials and process steps are described below.

Referring now to the drawing in which reference characters therein correspond to like numerals hereinafter, a particular composite formed of epoxy resins and fibrous materials is first shaped into a desired configuration 11, to be joined to a similar or like element 12 of the same epoxy nature, as shown in FIG. 1. Usually, a plurality of plies forming a laminate is included in such elements 11, 12. The shaping of such elements 11, 12 is not a part of the invention, as the final article of manufacture, including the composite joint, may vary extensively in dimensional characteristics, depending upon its purposes and applications, and such elements l1, 12 are fabricated in known manner prior to practice of the invention thereon. Each element l1, 12 comprises an epoxy matrix having fibers or filaments imbedded therein in a plurality of either unidirectional or multidirectional layers or plies. The multidirectional fibers in the layers or plies may be universally directed, insofar as practice of this invention is concerned. An end or terminal portion 13, 14 of each element 11, 12 is prepared by machining or by other conventional and known techniques, to include an angle and to thereby provide scarfed mating surfaces l5, 16, respectively; that is, as shown in FIG. 2, the end portions of the members 11, 12 are provided with surfaces pitched or bevelled longitudinally of their respective elements.

The scarfed end surfaces 15, 16 are pitched or shaped to converge towards the extremity of their respective elements, each preferably at an angle of 4, although this angle may vary. At best, machining techniques provide foronly 21 5 angle.

In prior practice, as shown in FIG. 2, the mere tips 17 of the imbedded fibers would be exposed, with no further alteration made to the faying surface of its element 11. The same would occur, of course, to the corre- 1 sponding element 12. Accordingly, the loads being transferred at a formed joint of such elements could pass only from one tip 17 in one adherend 11 to another tip in the other adherend 12.

In the practice of this invention, after the preparing step is completed, the elements 11, 12 are masked by suitable masking material (hereinafter, the maskant) except for the portions thereof which are to be chemically treated by the etchant. The maskants function is to withstand the effects of the etchant, and as manylayers as required to do so may be utilized. Various ways to mask the epoxy composite as a prelude to the etching step are available. For example, fluids which are acid resistant, such as those used in connectionwith chemical milling of metals, can constitute a maskant to be applied to either the entire specimen-to-be-etched or to the non-etched portion of the specimen which will come into contact with the etching fluid. In either case, after the maskant dries, it is then cut away in those areas of the specimen to be etched. Application of the maskant is performed by dipping the specimen in the fluid, or by spraying, and then drying the coated specimen in the open air. If necessary, multiple coats of maskant can be applied by repeating this technique. The portion of the specimen to be etched is then scribed and the maskant in that area peeled away. Following the etching step, which establishes the exposure of the fiber lengths, the remaining maskant is stripped or peeled off from the specimen and discarded.

Masking tapes also prove useful. Chemically resistant tapes, such as adhesive backed lead, aluminum, nylon, polyester, or similar suitable materials can be used to protect that portion of the specimen which is not to be etched.

In carrying out the etching step, portions of the elements 11, 12 including their mating surfaces 15, 16 are introduced into a bath for exposing, by chemical etching, lengths of fibers or filaments 19, 20 (FlGS. 4, 5 and 6) in their respective mating surfaces 15, 16. The etching bath may be contained within a metal tank lined with an organic plastic such as linear polyethelene or polypropylene lining, or with acid-proof brick and mortar. The elements 11, 12 may be supported in any suitable manner, such as by merely hanging from a wire, realizing that etching occurs uniformly inwardly from the scarfed mating surfaces. The results of such etching is to remove by dissolving the epoxy matrix from around and about a given length of all fibers or filaments and as shown in FIGS. 4 and 6. By this step, the available surface area of the fibers 19, 20 of both adherends l1, 12, for promoting their contiguity, is increased up to 300 percent, thus, ensuring substantially improved load carrying characteristics in the composite joint. An advantage resulting from this lies in the fact that the loads are transferred through the joint in the same plane as both of the adherends, thus, reducing the stress which heretofore resulted in high fatigue.

The lengths of the exposed adherend portions are preferably, as mentioned, 0.010 to 0.040 inch, although such exposed length is not limited to 0.040 inch. The amount of time in which the elements or adherends 11, 12 are immersed in the etchant is dependent upon the depth of the chemical etching of their matrices which is desired. Furthermore, a uniform etching results in the sense that the complementary angles of the scarfed mating surfaces 15, 16 are not materially or substantially altered. The results of the etching is a plurality of exposed lengths of fibers or filaments 19, 20 extending from the cooperating complementary ends-to-be-joined of the adherends 11, 12.

It may be noted here that the plurality of plies or layers, 21, 22, 23, 24, (FIG. 4) of fibers incorporated in the epoxy composite adherends may be unidirectional or multidirectional, as the case may be, and the process is applicable to one as equally as to the other.

A typical procedure used for etching a fiber-' reinforced epoxy composite'is described asfollowsr' l. The masked specimens are placed in a solution of 5 Other solutions may also be utilized where suitable.

2. The specimen is withdrawn from the bath and rinsed with tapwater to remove any residual acid.

3. The specimen is submerged in I a mild alkaline cleaner to neutralize all of the specimen. An example of such a cleaner is a non-silicated alkaline with good wetting or low surface tension properties.

4. The etched specimen is rinsed with tap water.

5. Any partially digested resin or residual insolubles or fluids is removed from the exposed fibers by airwater pressure blasting or ultrasonic wave clean- 6. The protective masking coating is removed and the total element is rinsed in tap water.

7. The specimen is dried to drive off any residual moisture.

As indicated by the aboveproced'ure, the etched adherends 1le,'l2e (FIG. 6) are flushed with a suitable neutralizer and water after removal from the etching bath. The neutralizer, of course, neutralizes any remaining acid while the water removes any chemicals sitting on the mating surfacesand fiber lengths. The etched and neutralized specimens are then subjected to a secondary cleaning step to remove any residual fluids or etched debris such as minute chunks of resin. The secondary cleaning step could be ultrasonic wave or water jet applications as required, by means of known devices providing such applications. The cleaned spec imens are then dried.

The next step is to lay up the joint being formed by the etched elements lle, 12e, whereby the fibers are encapsulated by the resin or epoxy matrix. In this stage of the process, an unsupported adhesive 25 in the form of a sheet is interposed (FIG. 7) between the etched scarfed mating surfaces 15e, 16e of such elements, and thence such elements are physically joined together.

The adhesive is unsupported in the sense that a fabric and holding these components together, withthe aid of tools and fixtures, are well known and used techniques in the art of laying up plastic materials.

Thereafter, the joint lay up is bonded,"such as; for ex-" ample, by or in vacuum bagging and autoclave pressure devices. These devices and techniques applicable to plastic articles being prepared for curing and bonding in such devices are well known in the trade and arts and need not be described here.

, What occurs during curing is as follows. Without a fabric carrier contained within the adhesive 25 to prevent the promotion of the contiguity of the exposed or is not contained within it to carry or support the resins of the adhesive. This is an important advantage, as will become apparent hereinafter. The insertion or positioning of the unsupported adhesive material 25 between such mating ends may be accomplished in known or conventional manner. The matching of the etched scarfed mating portions of the adherends as they are brought together against the inserted unsupported adhesive is sought, and is readily obtained by carefully placing together these components. Locating bared fibers in each adherend, the lengths of such fibers intermesh as the curing temperatures melt the adhesive. The pressure (vacuum) applied to the adherends causes the exposed or bared lengths of fibers in each adherend to intermesh with such lengths of the other adherend, increased fiber contiguity being assured across the bondline by the chemical etching which has produced sufficiently long lengths of such fibers. After setting up of the epoxy composite joint, the contiguous fibers, which are establishedby such intermeshing, are now able to transfer, across the bondline of the composite joint, structural loads the values of which were heretofore substantially less over those shown to be transferable in epoxy composite joints established by other than the practice of this invention and utilizing supported adhesives.

It should be understood that contiguity of the fibers does not necessitate a physical contact between the fibers of one adherend with the fibers of the other adherend. The resulting disposition of the fibers of both adherends across the bondline in the final manufacture which is sought may be likened to the disposition of the fibers which are disposed in their epoxy matrix for each adherend alone.

It should now be apparent that the limitations in the strength characteristics of prior epoxy composite joints Results obtained by utilization of the process, and as compared to results obtained in using known or conventional processes, are shown by the following examples:

7 The represents the tolerance in degrees by which the fibers may be angularly displaced relative to the longitudinal axis of the elements-to-be-joined, however, it should be understood that the invention may be practiced on laminate plies having universally or multidirectionally disposed fibers.

In obtaining the above results, the etching bath period extended to 12 hours, was maintained at 150F, and utilized a solution of hot sulfuric acid sodium dichromate mixture. It should be understood, however, that various factors may vary the etching step but which, nevertheless, do not limit the practice of the invention. These factors are: (1) the age of the etching solution, its varying concentrations, the temperature of the bath, and the nature of the epoxy being etched (some have fillers). For example, depending upon these factors, a range of the temperature contemplated is l30-250F, and the etching period may run up to as much as hours.

The curing cycle for joining the unsupported adhesives used in obtaining the above results was as follows;

Joint or Laminate No.

Unsupported Adhesive The following epoxy-based adhesives for example,

have been found to be usable; FM 123-5 and FM 1000 which are manufactured by and available from the Bloomingdale Rubber Company, Havre de Grace, Md., a division of American Cyanamid and Epon 9602,

.available from the Dexter Corporation, Franklin St., Olean, NY. 14760..

In regard to the aforementioned mixture, which was utilized in obtaining the indicated results, the range of its constituents is as follows: sulfuric acid, 34.0 to 40.0 oz./gal.; sodium dichromate, 3.6 to 10.2 oz./gal.; with the balance being demineralized water.

It should be understood that the maximum temperature and maximum concentration of such mixture, for the purposes of carrying out the practice of the invention, are limited only by' the nature of the maskant uti-' lized in the practice of the invention.

The execution of the method should be evident from the above description; however, briefly, its procedure is approached, if necessary, by an operator referring to a series-of-fiber-orientations plan which sets out how the fibers of an epoxy composite article-to-be-made are to be oriented in the development of such article. This,

of course, is not part of the invention, is standard procedure, and determines the configured natureof the final manufacture to-be-developed. Such configuration and in good or damaged condition. In the matter of repair work, of course, both fabricated and metallic but damaged components already exists. Thereafter, this invention is invoked to provide the joint that completes the final manufacture. The steps of preparing, scarfmg, masking, etching, and laying up with the unsupported adhesive, in accordance with the invention as described above. Depending on the shape of the formed joint, suitable locating and holding operations are applied thereto in the layup step. The joint is then bonded by known vacuum bagging or autoclave devices and procedures used therewith. I

In addition to graphite and boron fibers, glass, pitch blend, PRD- fiber 49 (a DuPont product), and carbon are examples of other inert fibers which may be utilized in the process. Examples of the nature of carbon and graphite yarns suitable for use in practicing this invention, although such practice is not limited thereto, are set out in US. Pat. No. 3,660,140, issued May 2, 1972. PRD-49 fibers are an E. I. DuPont de Nemours and Company, Inc. (Wilmington, Delaware 19898) high modulus organic fiber composition used for plastics reinforcement, and is available in the form of yarns and rovings for filament winding, collimated tapes and chopped fiber reinforcement, and as woven fabric for resin impregnation and lamination.

Examples of organic epoxy-based resins utilized to encapsulate the fibers, as well as being etchable about such fibers, are DLS and DLS 77, commercially available from the Ciba Chemical and Dye Co., Fairlawn, New Jersey. Similarly, the epoxy resin PR 279 is commercially available from its manufacturer, Minnesota Mining and Mfg. Co., St. Paul, Minnesota 55101 and the resin BP 919 and is commercially available from its maker, BloomingdaleRubber Co., supra. Ex-

amples of use for these fiber laminates include, but are not limited to, aircraft fairings, radomes, interior panels, rotor blades and filament wound pressure tanks.

The applications of this process are as innumerable as there are elements to-be-joined with their large variety of configurations, shapes and dimensions. For example, in joining an existing laminate to a new one, this process may be applied to the repair of a damaged section in an assembly or subassembly of parts, 'or to redesigning of an existing structure, or to extending the existing laminated article. In the case of joining a finished laminate to a metal part, the process has application to joining a wing skin to a wing joint or root end, or to join graphite to intermediate material to minimize galvanic corrosion. In this regard, the metallic element is cleaned in accordance with standards known in the particular industry. For example, a chemical cleaningon a metallic element (such as aluminum) to be joined to the fiber reinforced epoxy composite element provides the necessary abrading thereof for joining such elements together. Steel cleaning, of course, involves exemplified in a manner-so that it can be readily practiced by those skilled in the art to which it pertains, or

with which it is most nearly connected, such exemplification including what is presently considered to represent the best embodiment of the invention. However,

it should be clearly understood that the above descriptially equally exposed lengths of inert fibers disposed within and extending unidirectionally from its scarfed mating surface substantially parallel to the longitudinal axis of the elements-to-be-joined and being intermeshed with like lengths of fibers extending from the other adherend.

2. The improved fiber-reinforcedepoxy composite joint of claim 1 wherein said inert fibers are selected from the class of materials comprising graphite, boron, glass pitch blend, carbon, and high modulous organic fiber compositions.

3. The improved fiber-reinforced epoxy composite 'joint of claim 1 wherein said ends are scarfed at angles of from 4 and 6. 

1. AN IMPROVED FIBER-REINFORCED EPOXY COMPOSITE JOINT COMPRISING AT LEAST A PAIR OF FIBER-REINFORCED EPOXY COMPOSITE ADHERENDS HAVING ENDS SCARFED AND MATED IN COMPLEMENTARY RELATION TO EACH OTHER ACROSS AN UNSUPPORTED ADHESIVE, ALL BLONDED TOGETHER, EACH OS SUCH ADHERENDS HAING A PLURAITY OF LAYERS OF SUBSTANTIALLY EQUALLY EXPOSED LENGTHS OF INERT FIBERS DISPOSED WITHIN AND EXTENDING UNIDIRECTIONALLY FROM ITS SCARFED MATING SURFACE SUBSTANTIALLY PARALLED TO THE LONGITUDINAL AXIS OF THE ELEMENTS-TO-BE-JOINED AND BEING INTERMESHED WITH LIKE LENGTHS OF FIBERS EXTENDING FROM THE OTHER ADHEREND.
 2. The improved fiber-reinforced epoxy composite joint of claim 1 wherein said inert fibers are selected from the class of materials comprising graphite, boron, glass pitch blend, carbon, and high modulous organic fiber compositions.
 3. The improved fiber-reinforced epoxy composite joint of claim 1 wherein said ends are scarfed at angles of from 4* and 6*. 